102 Binary Tree Level Order Traversal

Original Description: Given a binary tree, return the level order traversal of its nodes’ values. (ie, from left to right, level by level)..

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• 要求： 给一个binary tree，逐层遍历

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• 返回：
  level order traversal

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• 例子：
  • input: [3,9,20,null,null,15,7]
  • output:
    [[3], [9,20], [15,7]]

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Solution

使用dfs,用level记录高度，根节点高度为0。

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
class Solution(object):
    def levelOrder(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        
        res = []
        self.dfs(root, 0, res)
        return res

    def dfs(self, root, depth, res):
        if root == None:
            return res
        if len(res) < depth+1:
            res.append([])
        res[depth].append(root.val)
        self.dfs(root.left, depth+1, res)
        self.dfs(root.right, depth+1, res)